1. Field of the Invention
This invention relates in general to a signal generating apparatus and associated operating method. More specifically, this invention relates to a signal generating apparatus and method that derives RFZC (RF zero crossing) signals adapted to an optical system.
2. Description of Related Art
An optical system accesses data from a disc by moving its optical pick-up head along the disc radial direction and simultaneously scanning the disc recorded surface by means of emitting laser beams projected onto the rotating disc. The optical pick-up head is driven for reading a target track from the rotating disc when the pick-up head focuses and tracks on the target one. Under the focusing operation, the objective lens moves along the direction perpendicular to the disc surface in order to correctly focus on the optical disc, while the objective lens also moves along the direction parallel to the disc surface and follows the target track for reading data under a following operation. Basically, a laser beam is firstly emitted to project onto the optical disc, while a photo sensor on the optical pick-up head is then used to receive the reflected laser beam. Signals associated with the data recorded on the optical disc or control signals used for servo controls can thus be easily derived from the information embedded in the reflected laser beam. Next, by using a pre-amplifier, those signals obtained by the optical pick-up head will be used for generating an RF signal, and signals for servo controls, such as RFRP (RF ripple) and RFZC (RF zero crossing) signals. Those signals are directed to a digital signal processor (DSP) for further processes.
FIG. 1 shows a phase relationship of the RF, RFRP and RFZC signals, wherein both of the RFRP signal and the RFZC signal are used in an optical system. The RF signal is a high frequency signal and is enveloped by an enveloping signal, which has an inverse phase relationship to the RFRP signal and has a frequency lower than the RF signal. An RFZC signal can be obtained by comparing the RFRP signal with a relative slice level (such as a dash line depicted in FIG. 1) that can be indicated by an average value of the RFRP signal. Therefore, the RFZC signal will be a positive value when the RFRP signal is larger than the slice level, while the RFZC signal will be a negative one when the RFRP signal is less than the slice level. Sometime the RFRP signal might drift from a normal level under certain situations. FIG. 2 shows a waveform diagram of the RFRP signal when the optical system varies from a following, seeking, and finally back to the following modes. In FIG. 2, the RFRP signal remains constantly before time point T0 under the following mode, and the peaks of RFRP signal drifts down from normal level at T0 at the beginning of the seeking mode. Furthermore, the RFRP signal turns constantly after T3 while seeking mode is finished. Obviously, if a constant voltage is used to slice the drifted RFRP signal under the seeking mode (between T0 to T3), more errors will be arisen and introduced into the duty cycle of the RFZC signal, which significantly affects the performance of the optical system.
A low-pass filter is used for obtaining the slice level conventionally, but the low-pass filter cannot catch the RFRP signal variations in the beginning of the seeking mode (i.e., at T0). Basically, the RF signal will be manipulated by a high-pass filter to filter out its DC level for further processing, but also unfortunately, to pass the high-pass filter will lower down the peak values of the RFRP signal. The above two effects will make the duty cycle of the RFZC signal vary out of 50 percent. Another conventional approach employs the DSP of an optical system to calculate required RFZC signal by averaging the peak-hold and bottom-hold values derived from the RFRP signal. The system performance of the optical system is therefore degraded since DSP load will be heavier than before. A further conventional approach employ hardware circuit to derive the RFZC signal, but this hardware circuit will be always activated to and provide RFZC values whatever the optical system needs, which obviously causes unnecessary calculation operations and power consumption. On the other hand, a glitch will appear in the obtained RFZC signal at the end of the seeking mode if the low-pass filter is used for obtaining the slice level, which still seriously affects the signal accuracy of the optical system. A need has been arisen to disclose an apparatus and related operating method for deriving an RFZC signal having 50% duty cycle and more adapted to an optical system so that the above disadvantages can be eliminated simultaneously.